Anisotropic Conductive Elastomer (ACE) is a composite of conductive metal particles in an elastomeric matrix. ACE is normally constructed such that it conducts along one axis only. Usually, ACE is a sheet that is made to conduct through its thickness. In one reduction to practice, ACE achieves its anisotropic conductivity by mixing magnetic particles with a liquid resin, forming the mix into a continuous sheet, and curing the sheet in the presence of a magnetic field. This results in the particles forming columns through the sheet thickness. The columns are electrically conductive. The resulting structure has the unique property of being flexible and anisotropically conductive. When a layer of ACE is compressed between two electrical conductors, the particles in the compressed column come into contact with each other and the conductors, forming an electrically conductive path.
As devices using ACE warm up, the elastomeric material thermally expands at a higher rate than the metal, ceramic and other components of the system. The lateral expansion of the elastomer disrupts the particle column integrity, and the vertical expansion tends to unload the columns, which expand at a much lower rate. Both of these effects impact on the electrical stability of the interconnect with temperature. It has been shown that the electrical instability caused by the differential thermal expansion of the elastomer can be eliminated by pre-stressing (stretching) the sheet to a level beyond that provided by the expansion of the elastomer over the operational thermal range. As a stretched sheet of elastomer is heated, it will relax with temperature but not physically move laterally until all the stress is gone. Hence no disruptive motion of the columns will occur over the system thermal operating range when sufficient stretch has been applied. Furthermore, as the sheet is stretched, the height of the columns does not change but rather the elastomer between the columns necks down to provide the material for the increased area of the stretched sheet. This necking provides the additional free volume needed by the system to offset the vertical expansion of the elastomer with temperature in a direction parallel to the columns.